Turbocharger and blade bearing ring therefor

ABSTRACT

A blade bearing ring for turbocharger application in diesel engines is described, which consists of an iron-based alloy with an austenitic basic structure having dendritic carbide precipitations.

The invention relates to a blade bearing ring for turbocharger applications, particularly in a diesel engine, according to the preamble of claim 1, and also to an exhaust gas turbocharger with a blade bearing ring, according to the preamble of claim 5.

Exhaust gas turbochargers are systems for increasing the power of piston engines. In an exhaust gas turbocharger, the energy of the exhaust gases is used for increasing the power. The power increase results from a rise in the mixture throughput per working stroke.

A turbocharger consists essentially of an exhaust gas turbine with a shaft and compressor, the compressor arranged in the intake tract of the engine being connected to the shaft, and the blade wheels located in the casing of the exhaust gas turbine and in the compressor rotating. In a turbocharger with variable turbine geometry, adjustable blades are additionally mounted rotatably in a blade bearing ring and are moved by means of an adjustment ring arranged in the turbine casing of the turbocharger.

The blade bearing rings have to satisfy extremely stringent material requirements. The material forming the blade bearing ring must be heat-resistant, that is to say still afford sufficient strength even at very high temperatures of up to about 900° C. Furthermore, the material must have high wear resistance and also corresponding oxidation resistance, so that the corrosion or wear of the material is reduced and, consequently, the resistance of the material under the extreme working conditions is still ensured.

A blade bearing ring of this type is known from DE 10 2004 062 564 A1. In this type of blade bearing ring, in contrast to the ferritic materials mostly used, an austenitic material is employed, an iron matrix alloy which has a high sulfur fraction for improving the lubricating action of the component. Owing to the specific composition, the creep resistance for the material is increased and therefore a high dimensional stability of the blade bearing ring at temperatures of above 850° C. is achieved.

By contrast, the object of the present invention is to provide a blade bearing ring according to the preamble of claim 1 and a turbocharger according to the preamble of claim 5 which have improved temperature and oxidation resistance and also corrosion resistance, which are distinguished by optimal tribological properties and which, moreover, exhibit a reduced susceptibility to wear.

The object is achieved by means of the features of claim 1 and of claim 5.

By virtue of the design according to the invention in the form of a blade bearing ring or an exhaust gas turbocharger comprising just such a blade bearing ring consisting of an austenitic iron-based alloy, a better temperature resistance of the material is achieved. This is also increased by a multiple by means of the dendritic carbide precipitations contained in the iron-based alloy. A blade bearing ring or exhaust gas turbocharger is thus provided, containing the blade bearing ring according to the invention which has an optimal temperature resistance in the range of up to 900° C., furthermore is highly heat-resistant, has high wear and corrosion resistance and, moreover, is distinguished by very good sliding properties, along with reduced oxidizability.

Furthermore, the blade bearing ring according to the invention remains dimensionally stable and therefore highly planar.

Without being involved in theory, it is presumed that carbide precipitations in the form of dendrites markedly increase the stability of the alloy material and therefore its strength on account of their unique structure. The fraction of dendrites in the material according to the invention amounts to a maximum of 20% by volume.

The maximum wear rate of the blade bearing ring according to the invention amounts to less than 0.08 mm for a bearing load of 10 to 18 N/mm², a sliding speed of 0.0025 m/s, a component temperature of 500 to 900° C., a surface roughness Rz of 6.3, a test duration of 500 h, a clock frequency of 0.2 Hz, an adjusting angle of 45°, a coefficient of friction of 0.28, a journal diameter of 4.77 mm, a pressure pulsation of >200 mbar, an exhaust gas pressure of >1.5 bar, and with diesel exhaust gas as the test medium.

During a thermal shock test run of 300 h, the component planeness of the blade bearing ring according to the invention amounts to less than 0.1 mm over a circumference of 80 mm.

The subclaims contain advantageous developments of the invention.

Thus, in one embodiment, the tribological properties of the blade bearing ring according to the invention can be improved by means of a corresponding content of manganese sulfide. As a result, wear due to sliding friction is markedly reduced, thus further increasing the durability of the blade bearing ring.

The addition of the elements niobium and vanadium may in this case further improve the iron-based alloy. Niobium is a carbide former or ferrite former and therefore increases the heat resistance and long-term rupture strength in the austenitic structure according to the invention. Vanadium, by contrast, refines the primary grain and therefore the cast structure. Vanadium, too, is a strong carbide former, with the result that the wear resistance of the material is increased. The grain refinement in this case leads to a higher dynamic surface pressure. Preferably, as components, niobium is present at 0 to 3.5% by weight and/or vanadium at 0 to 3.7% by weight in the iron-based alloy. The iron-based alloy may contain further elements, such as, for example, C, Cr, Ni, Mn, Si and N, in addition to iron.

According to a preferred embodiment, the iron-based alloy contains the following components C: 0.2 to 0.5% by weight, Cr: 16 to 22.5% by weight, Ni:5 to 14% by weight, Mn:5 to 15% by weight, Si ≦1.3% by weight, S: <0.5% by weight, Nb: 0 to 3.5% by weight, V: 0 to 3.7% by weight, N: 0.1 to 0.6% by weight, and Fe.

In a further embodiment, the blade bearing ring according to the invention is distinguished by a specific composition which contains the components:

-   -   C: 0.2 to 0.5% by weight     -   Cr: 16 to 22.5% by weight     -   Ni: 5 to 14% by weight     -   Mn: 5 to 15% by weight     -   Si: ≦1.3% by weight     -   S: <0.5% by weight     -   Nb: 0.75 to 3.5% by weight     -   V: 1 to 3.7% by weight     -   N: 0.1 to 0.6% by weight     -   and Fe.

The influence of the individual elements on an iron-based alloy is known, but it was then found, surprisingly, that exactly the combination described affords a material which, when processed into a blade bearing ring, gives this a particularly balanced property profile. Owing to this composition according to the invention, a blade bearing ring with especially high heat resistance and temperature resistance is obtained, which is distinguished by an outstanding sliding property and therefore especially low sliding wear. Moreover, the corrosion resistance is minimized, this also applying particularly to wet corrosion.

Thus, a material according to the invention produced in this way has the following properties:

Mechanical property Value Measurement method Tensile strength R_(m) >650 MPa ASTM E 8M/EN 10002-1; at increased temperature: EN 10002-5 Yield point R_(p 0.2) >270 MPa Standard method Elongation at break >12% Standard method Hardness 225-265 HB ASTM E 92/ISO 6507-1 Coefficient of 16-19 K⁻¹ Standard method linear expansion (20 to 900° C.)

According to a further embodiment of the invention, the blade bearing ring is free of sigma phases. This counteracts the embrittlement of the material and increases its durability. Sigma phases are brittle, sinter-metallic phases of high hardness. They arise when a body-centered cubic metal and a face-centered cubic metal, the atomic radii of which are identical with only a slight deviation, meet one another. Sigma phases of this type are undesirable because of their embrittling action and also on account of the property of the matrix to extract chrome. The material according to the invention is distinguished in that it is free of sigma phases. The embrittlement of the material is thus counteracted, and its durability is increased. The reduction or avoidance of the formation of sigma phases is achieved in that the silicon content in the alloy material is lowered to less than 1.3% by weight and preferably to less than 1% by weight.

Furthermore, it is advantageous to employ austenite formers, such as, for example, manganese, nitrogen and nickel, if appropriate in combination.

Claim 5 defines, as an independently handleable article, an exhaust gas turbocharger which, as already described, comprises a blade bearing ring which consists of an iron-based alloy with an austenitic basic structure having dendritic carbide precipitations.

FIG. 1 shows a perspective view, illustrated partially in section, of a turbocharger according to the invention. FIG. 1 illustrates a turbocharger 1 according to the invention which has a turbine casing 2 and a compressor casing 3 connected thereto via a bearing casing 28. The casings 2, 3 and 28 are arranged along an axis of rotation R. The turbine casing is shown partially in section, in order to make clear the arrangement of a blade bearing ring 6 and a radially outer guide blade cascade 18 which is formed by the latter and which has a plurality of adjusting blades 7 distributed over the circumference and having rotary axes 8. Nozzle cross sections are thereby formed, which are larger or smaller, depending on the position of the adjusting blades 7, and which act upon the turbine rotor 4, located in the center on the axis of rotation R, to a greater or lesser extent with the engine exhaust gas supplied via a supply duct 9 and discharged via a central connection piece 10, in order via the turbine rotor 4 to drive a compressor rotor 17 seated on the same shaft.

In order to control the movement or position of the adjusting blades 7, an actuating device 11 is provided. This may per se be of any desired design, but a preferred embodiment has a control casing 12 which controls the control movement of a tappet member 14 fastened to it, in order to convert this movement on an adjusting ring 5 located behind the blade bearing ring 6 into a slight rotational movement of said adjusting ring. Between the blade bearing ring 6 and an annular part 15 of the turbine casing 2, a free space 13 for the adjusting blades 7 is formed. So that this free space 13 can be safeguarded, the blade bearing ring has spacers 16.

EXAMPLE

An alloy, from which a blade bearing ring according to the invention was formed, was produced from the following elements according to a customary method. The chemical analysis yielded the following values for the elements: C: 0.2 to 0.5% by weight; Cr: 17 to 21% by weight; Ni: 5.5 to 10.5% by weight; Mn: 7.5 to 11% by weight; Si: max. 1% by weight; S: <0.5% by weight; Nb: 0.75 to 1.7% by weight; N: 0.1 to 0.5% by weight; V: 1 to 1.9% by weight; the rest: iron.

The blade bearing ring produced according to this example was distinguished by a tensile strength R_(m) of 655 MPa (ASTM E 8M/EN 10002-1; at increased temperature: EN 10002-5). The yield point R_(p) 0.2 (measured according to standard methods) amounted to 276 MPa. The elongation at break of the material (measured according to standard methods) amounted to 13.2%. The hardness of the material (measured according to ASTM E 92/ISO 6507-1) amounted to 243 HB. The coefficient of linear expansion (measured according to standard methods) amounted to 17.6 K⁻¹ (20 to 900° C.). The material was subjected to a validation test series which comprised the following tests:

-   -   outdoor exposure test     -   changing climate test     -   thermal shock test/cycle test—300 h     -   hot gas corrosion test in a fission furnace

The component was distinguished in all the tests by excellent resistance to the acting forces. The material therefore had extremely high wear resistance and outstanding oxidation resistance, so that corrosion or wear of the material under the specified conditions was markedly reduced and therefore the resistance of the material was still ensured even over a long period of time.

Thermal Cycle Test:

The component according to the invention was subjected to a thermocycle test, the thermal shocks being operated as follows:

-   1. use of stationary rotors; -   2. 2-turbocharger operation; -   3. test duration: 350 h (approximately 2000 cycles); -   4. during the entire test, the exhaust gas flap in the turbochargers     remains open at 15°; -   5. high temperature: nominal power point T3=750° C., turbocharger     mass flow on the turbine side: 0.5 kg/s; -   6. low temperature: T3=100° C., turbocharger mass flow on the     turbine side: 0.5 kg/s; -   7. cycle duration: 2×5 min. (10 min.); -   8. execution of three intermediate crack tests.

LIST OF REFERENCE SYMBOLS

-   1 Turbocharger -   2 Turbine casing -   3 Compressor casing -   4 Turbine rotor -   5 Adjusting ring -   6 Blade bearing ring -   7 Adjusting blades -   8 Rotary axes -   9 Supply duct -   10 Axial connection piece -   11 Actuating device -   12 Control casing -   13 Free space for guide blades 7 -   14 Tappet member -   15 Annular part of the turbine casing 2 -   16 Spacer/spacing boss -   17 Compressor rotor -   18 Guide blade cascade -   28 Bearing casing -   R Axis of rotation 

1. A blade bearing ring for a turbocharger application, in particular in diesel engines, consisting of an iron-based alloy with an austenitic basic structure having dendritic carbide precipitations.
 2. The blade bearing ring as claimed in claim 1, which contains manganese sulfide.
 3. The blade bearing ring as claimed in claim 1, which contains as further components niobium at 0 to 3.5% by weight and/or vanadium at 0 to 3.7% by weight.
 4. The blade bearing ring as claimed in claim 1, which contains the following components: C: 0.2 to 0.5% by weight, Cr: 16 to 22.5% by weight, Ni: 5 to 14% by weight, Mn: 5 to 15% by weight, Si: ≦1.3% by weight, S: <0.5% by weight, Nb: 0.75 to 3.5% by weight, V: 1 to 3.7% by weight, N: 0.1 to 0.6% by weight and Fe.
 5. The blade bearing ring as claimed in claim 1, which is free of sigma phases.
 6. An exhaust gas turbocharger for diesel engines, comprising a blade bearing ring consisting of an iron-based alloy with an austenitic basic structure having dendritic carbide precipitations.
 7. The exhaust gas turbocharger as claimed in claim 6, wherein the alloy contains manganese sulfide.
 8. The exhaust gas turbocharger as claimed in claim 6, wherein the alloy which contains as further components niobium at 0 to 3.5% by weight and/or vanadium at 0 to 3.7% by weight.
 9. The exhaust gas turbocharger as claimed in claim 6, wherein the alloy contains the following components: C: 0.2 to 0.5% by weight, Cr: 16 to 22.5% by weight, Ni: 5 to 14% by weight, Mn: 5 to 15% by weight, Si: ≦1.3% by weight, S: <0.5% by weight, Nb: 0.75 to 3.5% by weight, V: 1 to 3.7% by weight, N: 0.1 to 0.6% by weight and Fe.
 10. The exhaust gas turbocharger as claimed in claim 6, wherein the alloy is free of sigma phases. 